Fin Stabilizers
Courtesy of Arcturus Marine
Irrespective of other placement criteria, fins must be installed within the “beam envelope” in order to ensure that they are as
protected as possible during groundings and when coming along side bulkheads or pilings.
each side of the vessel’s bottom—operate in unison to
dampen or eliminate roll. (Larger vessels may have two
fins on each side.) The fins move in directional unison;
that is, they turn clockwise or counterclockwise
simultaneously. In doing so, they induce lift that, when
applied at the right moment and for a specific duration,
counteracts the vessel’s roll.
For instance, if the vessel begins to roll to starboard,
the starboard fin turns clockwise; this creates lift on the
starboard side. The port fin also turns clockwise, pulling
the vessel down on that side. These actions, which
occur in the blink of an eye, effectively counteract the
roll before it can take hold of the boat. Under most
conditions, the effect of the wave and the counter-effect
of the stabilizers occur without the crew noticing.
In order for fin stabilizers to operate properly, the
vessel must be making headway (the more, the better).
Because the fins must generate lift much like the wing
of an aircraft, movement of water over the foil-shaped
surface is imperative. The exception is stabilizers that
are specifically designed to operate while the vessel is
at rest. (Several manufacturers offer these.)
This explanation makes the process sound simple and,
in theory, it is. But making it all happen in a reliable,
effective, and safe manner requires a considerable amount
of effort on the part of engineers, fabricators, computer
programmers, circuit designers, and hydraulic, mechanical,
electrical, metal, and fiberglass technicians, not to mention
untold hours of trial and experimentation. While some
stabilization systems use compressed air or electric motors
for actuation of the fins, nearly all systems used aboard
small and medium-size recreational and commercial
vessels utilize hydraulics. Hydraulic actuation enables the
stabilizer system to react quickly and with sufficient torque
to influence the movement of a large, heavy, fast-moving
vessel. The fins and the method by which they move,
however, are only a small part of the story.
Sensors or gyroscopes measure a vessel’s roll velocity,
angle, and acceleration hundreds or thousands of times a
second (the more information polling, the better), and the
information gathered is sent to a computer, where it
is crunched using sophisticated and closely guarded
proprietary algorithms. The product of that analysis is
then sent in the form of electronic signals to servos
that control the fins’ hydraulic pistons or actuators. The
speed at which this occurs is critical. The effectiveness of
the system is predicated on the lightning-quick reaction
of sensors, analysis of information, and actuation of fins.
In some cases, fins are capable of moving through their
full arc, as much as 45 degrees, in less than a second.
SYSTEM COMPONENTS
The components that make up the average fin
stabilizer system are many and varied. Of course, all
stabilizer systems include fins (the portion that can
be seen from outside the vessel), but not all fins are
alike. Determining the proper fin size is a function of
several factors, including the vessel’s waterline length,
displacement, transverse metacentric height (also known
as “GM”), beam, and desired cruising speed (this is the
speed at which the fins will work optimally, which is
critical). If two vessels are the same length but one
